The Mind of the Mnemonist, A. R. Luria

Synesthesia (also spelled synæsthesia or synaesthesia; from the Ancient Greek σύν syn, “together”, and αἴσθησις aisthēsis, “sensation”) is a neurological phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway.[1][2][3][4] People who report such experiences are known as synesthetes.

Difficulties have been recognized in adequately defining synesthesia:[5][6] Many different phenomena have been included in the term synesthesia (“union of the senses”), and in many cases the terminology seems to be inaccurate. A more accurate term may be ideasthesia.

In one common form of synesthesia, known as grapheme → color synesthesia or color-graphemic synesthesia, letters or numbers are perceived as inherently colored.[7][8] In spatial-sequence, or number form synesthesia, numbers, months of the year, and/or days of the week elicit precise locations in space (for example, 1980 may be “farther away” than 1990), or may appear as a three-dimensional map…